How material sensory properties and individual differences influence the haptic aesthetic appeal of visually presented stimuli

Touch plays a crucial role for humans. Despite its centrality in sensory experiences, the field of haptic aesthetics is underexplored. So far, existing research has revealed that preferences in the haptic domain are related to stimulus properties and the Gestalt laws of grouping. Additionally, haptic aesthetics is influenced by top-down processes, e.g., stimulus familiarity, and is likely to be modulated by personality and expertise. To further our understanding of these influences on haptic aesthetic appraisal, the current study investigated the imagined haptic aesthetic appeal of visually presented material surfaces, considering the role of haptic expertise, Need for touch, personality traits. The results revealed a positive influence of familiarity, simplicity, smoothness, warmth, lightness, dryness, slipperiness and a negative influence of complexity on individuals' aesthetic responses. While the study failed to support the predicted influence of Need for touch and haptic expertise on aesthetic responses, results did reveal an influence of openness to experience, conscientiousness and neuroticism. Despite the limitations related to the indirect stimuli presentation (vision only), the findings contribute to the relatively unexplored role of bottom-up and top-down features in haptic aesthetics that might be incorporated into the design of consumers’ products to better meet their preferences.


How material sensory properties and individual differences influence the haptic aesthetic appeal of visually presented stimuli
Marella Campagna 1* & Rebecca Chamberlain 2 Touch plays a crucial role for humans.Despite its centrality in sensory experiences, the field of haptic aesthetics is underexplored.So far, existing research has revealed that preferences in the haptic domain are related to stimulus properties and the Gestalt laws of grouping.Additionally, haptic aesthetics is influenced by top-down processes, e.g., stimulus familiarity, and is likely to be modulated by personality and expertise.To further our understanding of these influences on haptic aesthetic appraisal, the current study investigated the imagined haptic aesthetic appeal of visually presented material surfaces, considering the role of haptic expertise, Need for touch, personality traits.The results revealed a positive influence of familiarity, simplicity, smoothness, warmth, lightness, dryness, slipperiness and a negative influence of complexity on individuals' aesthetic responses.While the study failed to support the predicted influence of Need for touch and haptic expertise on aesthetic responses, results did reveal an influence of openness to experience, conscientiousness and neuroticism.Despite the limitations related to the indirect stimuli presentation (vision only), the findings contribute to the relatively unexplored role of bottom-up and top-down features in haptic aesthetics that might be incorporated into the design of consumers' products to better meet their preferences.
Touch is a crucial sensory modality and given its powerful affective and cognitive components it is an important conduit for pleasurable and rewarding experiences 1 .Our sense of touch emerges early in ontogeny 2,3 , is characterized by the widest bodily distribution of sensory receptors relative to any other sense 4 , and remains the last sense to deteriorate with the aging process 5 .Due to its reciprocal character-active (haptic) and passive (tactile)-touch is crucial for fostering social bonds through oxytocin 6,7 release and maintaining psychological well-being via interpersonal stimulation 8,9 .This effect is mediated by C-tactile fibers, slow-conducting afferents located in hairy skin regions, which are specifically responsive to the affective dimensions of touch 10,11 .
For the present study purpose, we focused on the imagined "haptic" feel of materials, involving the "imagined" dynamic, interactive processes that combine sensing, influencing through touch, as opposed to the purely perceptual and passive imagined "tactile" feel.
Despite its centrality to survival, and its intrinsic capacity to stimulate arousal, little is known about the hedonic aspects of haptic stimulus processing.
So far, the field of haptic aesthetics has revealed some underlying preferences in relation to stimulus properties, which are shared by other sensory modalities.The Gestalt principles influencing perceptual grouping (e.g., similarity, proximity, closure) affects the perception of haptic stimuli in both unimodal, and cross-modal settings 12,13 and individuals seem to prefer complex order (unity in variety) in the haptic domain 14 .Additionally, the occurrence of the Aesthetic Aha phenomenon 15 , whereby individuals experience a pleasurable sense of reward in perceptually challenging circumstances, also takes place in the haptic domain.It was observed that liking judgments were significantly associated with situations under which both interest (given by high complexity) and pleasantness (given by strength of insight) were high.However, it is worth noting that haptic pleasantness was also associated with the material feel.It can be inferred that when tactile aesthetic judgments are to be made, individuals favor an optimal balance between fluent perceptual processing, provided by a sense of the whole,

The present research
In light of the extant body of research within the field of haptic aesthetics, the present study aims to more fully explore the haptic aesthetic appeal of visually presented materials surfaces, taking into account the impact of individual factors.To this end, a set of eighteen materials varying in typicality, and presented in the form of brief videos, were shown to a sample of participants varying in haptic expertise.Imagined perceptions of the material properties, complexity, familiarity, and haptic aesthetic responses were assessed via semantic differential scales.Individual differences in haptic expertise, personality traits and Need for touch were also measured.The study was designed to address the following research questions: 1. To what extent do materials' sensory properties (e.g., smoothness vs. roughness) and individual subjective perceptions of complexity and familiarity influence the imagined haptic aesthetic appraisal of visually presented stimuli?
2. To what extent do individual differences in haptic expertise, Need for touch and personality traits influence the imagined haptic aesthetic appraisal of visually presented stimuli?
It was hypothesized that a material's smoothness, slipperiness, and perceived familiarity would positively predict all facets of imagined haptic aesthetic appraisal in line with existing research conducted in uni-modal, bi-modal settings 21,22,24,32,51 .Moreover, it was predicted that a material's perceived complexity would positively affect participants' ratings of interest, in line with existing aesthetic theory 16 .Furthermore, given the top-down nature of haptic stimulus processing, and the presence of differences in motivational components relative to haptic exploration, it was hypothesized that individual differences-haptic expertise, Need for touch, openness to experience-would account for variations in levels of interest, and haptic invitation [35][36][37][38] .Specifically, high levels of haptic expertise, trait openness to experience and Need for touch were predicted to affect imagined haptic aesthetic ratings.

Data preparation
First, we screened the data and excluded participants with large amounts of missing data, extremely short (< 720 s) or long completion times (> 7200 s) or responses polarized around the mean value on all semantic differential scales (SD < 1 around mean 3.8-4.2),leading to the exclusion of 188 participants.All variables were then inspected for univariate and multivariate outliers using p < 0.001 criterion for Mahala Nobis distance, and eleven participants were excluded on this basis, leaving a final sample of N = 347, representing a 36.45%loss of data, but exceeding the target sample size.
Results from assumptions of normality using the Shapiro-Wilk test, suggested several violations, nevertheless, all dimensions reported normal Q-Q plots of standardized residuals, and values for skewness and kurtosis within the acceptable limits, thereby no data transformations were applied.Residual and scatter plots indicated that assumptions of normality, homoscedasticity and linearity were all satisfied-few variables presented a high degree of association with each other.However, the Flat/Bumpy scale was excluded from further analysis due to its high level of collinearity with the Smooth/Rough scale 64 .

Descriptive statistics
Means and standard deviations for haptic expertise, Need for touch and personality traits (Big Five Personality traits "BFI") are given in Table 1.Figures 1 and 2 show the distribution of ratings for the perceived sensory properties of each material.As can be seen from these plots, glass, faux-fur and silk were judged as the smoothest and most slippery materials (Fig. 1).Dryness ratings for the materials showed relatively less variance, with clay being rated the least dry and sandpaper, lace and cashmere the driest (Fig. 1).Tweed, fur and cashmere were considered the warmest materials, with steel, clay and glass being the coolest.Crinoline, lace and silk were rated as the lightest materials, while steel and wood were rated as relatively heavier (Fig. 2).With regard to the haptically imagined aesthetic qualities of the materials, while there was some variability in how different stimuli were evaluated, sandpaper and concrete were consistently considered the least interesting, inviting, pleasant, evocative and beautiful (Figs. 3 and 4).On the other hand, silk, lace, fur and cashmere were consistently found to be the most interesting, inviting, pleasant, evocative, comfortable and beautiful relative to the other materials (Figs. 3 and 4).

Generalized linear mixed effects analyses
To investigate whether individual differences in personality traits, expertise and materials' sensory properties predicted participants' haptic aesthetic appeal of visually presented stimuli, we performed a series of generalized linear mixed effect analyses, modeling for variability at the stimulus and participant level.All analyses in the present research were conducted in R version 3.3.3.Significance was calculated using the lmerTest package 65 , which applies Satterthwaite's method to estimate degrees of freedom and generates p-values for mixed models.The dependent measures of these analyses were subjective ratings of imagined haptic aesthetic appreciation: Interest, Pleasantness, Comfort, Invitingness, Beauty and Evocativeness.For all the analyses, subjects and materials were included as random intercepts.www.nature.com/scientificreports/

Linear mixed effects models of Sensory properties, Familiarity, and Complexity on Aesthetic responses
The maximum random effect's structure justified by the data, contained random intercepts for participants and materials, but no random slopes: P-values were obtained by likelihood ratio tests of the full model with the effect in question against the model without the effect in question: Interest/Pleasantness/Comfort/Beauty/Evocativeness/Invitingness ~ 1 + (1|Subject) + (1|Material).Visual inspection of residual plots did not reveal any obvious deviations from homoscedasticity or normality.
The generalized linear mixed effect model for ratings of interest (Table 2) revealed positive effects of three sensory properties: perceived warmth, smoothness and lightness, and a negative effect of perceived dryness.Additionally, there was a significant positive effect of familiarity and complexity on interest ratings.Pleasantness and evocativeness ratings (Table 2 and 7), were significantly predicted by perceived warmth, smoothness, familiarity, lightness and slipperiness (Tables 3, 4, 5).Conversely, a reversed relationship was reported for perceived    www.nature.com/scientificreports/complexity, which was negatively related to pleasantness and evocativeness ratings.Finally, in the case of comfort, beauty and invitingness ratings (Tables 4, 6 and 7), the models showed significant positive effects of smoothness, familiarity, warmth, lightness and a negative effect of perceived dryness.Across all models, random effects for subjects were relatively large, compared with random effects for stimuli, meaning that there was a greater variance in response between participants than between different materials.In sum, the results suggest that the materials'      The maximal random-effects structure that converged included random intercepts for participants and materials, but no random slopes: P-values were obtained by likelihood ratio tests of the full model with the effect in question against the model without the effect in question: Interest/Pleasantness/Comfort/Beauty/Evocativeness/Familiarity/Invitingness ~ 1 + (1|Subject) + (1|Material).Visual inspection of residual plots did not reveal any obvious deviations from homoscedasticity or normality.
The generalized linear mixed effect model for ratings of interest (Table 8) revealed a significant positive effect of conscientiousness.Pleasantness ratings (Table 9) were significantly predicted by openness to experience and conscientiousness.In the case of comfort ratings (Table 10), the model revealed a significant positive effect of conscientiousness, and negative effects of trait Openness to experience and Neuroticism.For invitingness ratings (Table 11), the fit of the overall model in relation to the null was non-significant.Beauty ratings (Table 12) were positively predicted by conscientiousness and Neuroticism.Finally, evocativeness ratings (Table 13) were positively predicted by openness to experience and conscientiousness.Across all models, random effects for both subjects and materials were relatively large.

Discussion
The primary purpose of the current study was to examine the influence of materials' sensory properties and individual differences on the haptic aesthetic appeal of visually presented stimuli.Considering the ratings provided by participants on the imagined perception of material properties, we found that silk, glass, faux-fur were www.nature.com/scientificreports/considered as the smoothest and most slippery materials, while sandpaper was rated as the driest, roughest and least slippery.These findings align with existing research adopting both haptic unimodal, and bimodal visuotactile conditions 20,21,32,51 .Furthermore, aesthetic responses marked out silk, lace and fur as the most preferred stimuli in terms of beauty, comfort and interest ratings, while sandpaper and concrete were often rated the lowest in these dimensions.www.nature.com/scientificreports/Linear mixed effects analyses revealed consistent and significant associations between imagined material properties and aesthetic responses, as well as between individual differences and aesthetic responses, although to a weaker degree.Across the models tested we saw a positive effect of familiarity, warmth, smoothness, and lightness on aesthetic responses to the materials, supporting existing research and theory that suggests that the apparent safety, familiarity and comfort of a material drives haptic aesthetic responses to that material 32 .In particular, the perception of a given material's warmth may have potentially triggered implicit memories of trust, comfort, or prenatal experience within the maternal womb-given the early ontogeny of the haptic modality 2,3,66-68 .Preference for safe, familiar stimuli may be an innate propensity or the result of acquired knowledge and experience 69 , highlighting a role for both implicit, explicit memories in individuals' aesthetic judgments of haptic materials, and the top-down nature of haptic aesthetic appreciation.The current study is unable to speak directly to the question of at what stage such preferences for haptic stimuli emerge, and developmental research would be a valuable way to further explicate the mechanisms underlying these preferences.
In the current study, complexity ratings for the materials positively predicted interest ratings but negatively predicted pleasantness, invitingness, and evocativeness ratings.This aligns with the findings of Berlyne 16 in the visual, auditory domains in which complexity differentially predicted interest and aesthetic pleasure based on mechanisms of arousal.Interest, and pleasure are generally described as distinct routes to liking, pleasure being a hedonic response in automatic processing, whereas interest is linked to an increase in the desire to explore stimuli 15 .Given that haptic exploration not only stems from autotelic but also from discovery purposes, a certain degree of complexity might be crucial in providing an optimal level of arousal in haptic experience.However, the present study did not aim to explicitly address participants' aesthetic responses towards materials that were systematically varied in terms of their complexity.Additionally, the measurement of complexity was only derived subjectively rather than considering the objective complexity of each stimulus.Future research may consider investigating whether different levels of complexity produce variation in participants' perceived interest and pleasantness, and whether that interacts with relevant individual differences (e.g., trait curiosity and openness to experience).
Turning to the analysis of individual differences in relation to imagined haptic aesthetic appreciation, personality traits have been previously shown to affect the way sensory, emotional/affective information is processed across domains [36][37][38] .This appears to be the case in the present study, where openness to experience, neuroticism, and conscientiousness predicted facets of imagined haptic aesthetic appreciation.Specifically, openness to experience and conscientiousness positively predicted pleasantness and evocativeness ratings, while openness to experience negatively predicted comfort ratings.Additionally, conscientiousness positively predicted interest and comfort ratings.Much research has indicated that openness to experience relates to aesthetic sensitivity 70 .Highly open individuals display a more positive aesthetic attitude and increased receptivity to sensory stimuli 71,72 .This theoretical account potentially explains the observed positive relationship emerged with ratings of pleasantness and evocativeness.Nevertheless, it is unclear why trait openness to experience failed to significantly positively correlate with all other facets of haptic aesthetic appreciation.The fact that participants could not explore the materials using the haptic modality in the current study may have introduced a source of bias in the interpretation of the adjectives used to aesthetically evaluate the materials, which then potentially interacted with some of the personality traits explored in the current study.Amongst all big five personality traits investigated in the present study, conscientiousness appeared as the strongest predictor of facets of haptic aesthetic appreciation.Trait conscientiousness refers to an individual's level of persistence, organization, dependability, self-discipline and goal-oriented behaviors.Within aesthetic research, conscientiousness has often displayed weak or null patterns of association with aesthetic experiences [73][74][75][76] .A possible reason that we observed an effect of conscientiousness in the current study may be the phenomenon of "extreme response style".In questionnaire-based studies, and in the presence of high levels of extraversion and conscientiousness, it was observed that participants are keen to favor extreme response categories, thereby resulting in possible correlation between ratings 49,77-79 .www.nature.com/scientificreports/

Measures
To investigate participants' imagined perceptions of sensory, aesthetic qualities of the materials, as well as their perceived familiarity and complexity, a 7-point semantic differential scale was administered, including fourteen antonym pairs of adjectives.The semantic differential scales were drawn from the Sensory Emotional scale of Touch perception 58 , and the existing haptic literature exploring touch perception and surface properties 21,34,59,60 .This yielded six items related to aesthetic appreciation, six items related to the material's sensory properties, and two additional items to assess perceived familiarity and complexity of the material.Words on each semantic differential scale were randomly polarized and presented in a random order.The antonym pairs related to the perceived sensory properties of the material were: smooth/rough; warm/cold; dry/wet; flat/bumpy; slippery/ sticky and light/heavy.Antonym pairs related to the aesthetic qualities of the material were: interesting/boring; pleasant/unpleasant; comfortable/uncomfortable; beautiful/ugly; inviting/repelling; evocative/not evocative.Additional antonym pairs relating to complexity and familiarity were: complex/simple; familiar/unfamiliar.Individual differences were assessed focusing on three dimensions: the participant's experience with tasks that require high haptic ability (expertise) and haptic efficiency (manual dexterity); the natural tendency to engage in haptic explorations for aesthetic purposes (Need for touch); and personality traits.To capture each participants' haptic expertise and manual dexterity, a compacted and revisited version of the Touch Experience Questionnaire developed by Guest et al. 58 was used, in which participants reported musical instruments and type of music they played, artistic experiences, hobbies involving touch, and the extent to which their job-domain required haptic ability.Ratings were made via selecting which of the five levels of experience (ranging from 5 = expert, and 0 = No experience) described the participant's level of familiarity or skills for the items in the scale sections.To identify individual differences in participants' motivation to touch objects or materials for hedonic purposes, known as Need for touch, the Need for touch (NFT) scale developed by Peck and Childers 35 was employed.However, for the present study purposes, only six items pertaining to the Autotelic NFT subscale were used, measured on an adjusted 5-point Likert scale (anchored to 1 = Strongly disagree, and 5 = Strongly agree).Finally, participants' personality traits were measured using The Big Five short inventory questionnaire (BFI) 61 , consisting of fifteen items, measured on a 5-point Likert scale (1 = Strongly disagree, 5 = Strongly agree).www.nature.com/scientificreports/

Procedure
The experiment was conducted remotely, using the online software platform Qualtrics-each participant could partake in the study using a laptop or tablet.Participation was on a voluntary basis.All data, including age, gender, nationality and language spoken were recorded through the platform software.The experiment consisted of three blocks of six video trials, each block followed by a questionnaire.There was no time limit for completion of the study.The whole procedure lasted approximately 35 min per participant.
In each trial participants were presented with a video showing a human hand manipulating a material, lasting for approximately 10 s.If required, participants could see the video repeatedly.Alongside the video, a 7-point semantic differential scale with fourteen items was displayed.Participants were instructed to imagine exploring the material's surface with their hands, and then rate it on the list of attributes provided (see materials).In situations where touch is not available, giving participants the instruction to "mentally explore" the material, may promote volitional mental imagery of touch, further strengthening "a simulation of touch", already elicited by the sight of a touch percept 55,56,62,63 .At the end of Block 1 participants completed the Touch Experience questionnaire 25 , in Block 2 they completed the BFI short inventory 61 and in Block 3 they completed the Need for touch Questionnaire 35 .

Table 1 .
Means and standard deviations for haptic expertise, need for touch and BFI scores.

Table 2 .
Generalized linear mixed effect model of material properties on interest ratings.In the table * indicates p < 0.05; ** indicates p < 0.001.

Table 3 .
Generalized linear mixed effect model of Material properties on Pleasantness ratings.In the table * indicates p < 0.05; ** indicates p < 0.001.

Table 4 .
Generalized linear mixed effect model of material properties on evocativeness ratings.In the table * indicates p < 0.05; ** indicates p < 0.001.

Table 5 .
Generalized linear mixed effect model of material properties on comfort ratings.In the table * indicates p < 0.05; ** indicates p < 0.001.

Table 6 .
Generalized linear mixed effect model of material properties on invitingness ratings.In the table * indicates p < 0.05; ** indicates p < 0.001.

Table 7 .
Generalized linear mixed effect model of material properties on beauty rating.In the table * indicates p < 0.05; ** indicates p < 0.001.

Table 10 .
Generalized linear mixed effect model of personality traits on comfort ratings.In the table * indicates p < 0.05; ** indicates p < 0.001.

Table 11 .
Generalized linear mixed effect model of personality traits on invitingness ratings.In the table * indicates p < 0.05; ** indicates p < 0.001.

Table 12 .
Generalized linear mixed effect model of personality traits on beauty ratings.In the table * indicates p < 0.05; ** indicates p < 0.001.

Table 13 .
Generalized linear mixed effect model of personality traits on evocativeness ratings.In the table * indicates p < 0.05; ** indicates p < 0.001.